ETHANOL INHIBITION OF RECOMBINANT HETEROMERIC NMDA CHANNELS IN THE PRESENCE AND ABSENCE OF MODULATORS

The NMDA receptor/channel has been shown to be inhibited by ethanol in the clinically relevant range 25-100 mM. We studied heteromeric assem blies (NR1b/NR2) of the NMDA receptor, expressed in oocytes, to addres s three questions regarding this inhibition, and discovered the follow ing: (1) The inhibition was nearly equivalent when ethanol was coappli ed with the agonist, and when ethanol was introduced after steady-stat e current was established, suggesting that ethanol does not act by int erfering with the activation process of the NMDA receptor. (2) The deg ree of inhibition was controlled by the NR2 subunit, with both NR2A an d NR2B significantly more sensitive to ethanol than NR2C and NR2D. (3) Manipulation of the NMDA receptor with a number of agents that normal ly modulate it did not alter the degree of inhibition produced by etha nol. The presence of Mg2+ (3 and 12.5 mu M), Zn2+ (1 and 10 mu M), or the glycine antagonist 7-chlorokynurenic acid (1.25 or 5 mu M), did no t alter the ethanol sensitivity of heteromeric (NR1b/NR2A, NR1b/NR2B, NR1b/NR2C) NMDA receptors. Redox modulation of the NMDA receptor by di thiothreitol (2 mM) or 5,5'-dithiobis(2-nitrobenzoic acid) (1 mM) also did not alter the degree to which ethanol inhibits NMDA receptors. Ta ken together, these results indicate that the ethanol sensitivity of n ative NMDA receptors, which likely exist in heteromeric form, results from actions at a site different from those of known modulators of the receptor.